X Digital Subscriber Line (xDSL) is a technology for high speed data transmission on a telephone twist pair, that is, an Unshielded Twist Pair (UTP). Except for the DSL based on baseband transmission, the xDSL based on passband transmission utilizes the frequency-division multiplexing technology, so that the xDSL and the Plain Old Telephone Service (POTS) coexist on the same twist pair. The xDSL occupies the high frequency band, and the POTS occupies the baseband part below 4 KHz. The POTS signals and the xDSL signals are separated by a splitter. The xDSL based on passband transmission uses the Discrete Multitone Modulation (DMT).
With the improvement of the frequency band used by the xDSL technology, the crosstalk, especially the crosstalk in the high frequency band, becomes prominent. The uplink and downlink channels of xDSL use the frequency-division multiplexing. The Near-End CrossTalk (NEXT) may hardly affect the performance of the system, while the Far-End CrossTalk (FEXT) will affect the transmission performance of the line seriously. As a result, when a plurality of subscribers in the same cable requires activating the xDSL service, the FEXT may cause a low speed on some lines, an unstable performance, or even an unactivable service. The outgoing line ratio of a DSL access multiplexer (DSLAM) is low.